Near‐Infrared Fluorescence of Novel Pyridinium−Cyclic Enolate Betaine Dyes π‐Expanded by Condensed Cyclic Thiophene Spacers

Herein, we report the synthesis of new near‐infrared (NIR) fluorescent dyes based on the pyridinium−cyclic enolate betaine (PCB) skeleton. The formation of the electron‐donor−π−electron‐acceptor (D−π−A) type dye with the electron‐donating N,N ‐diphenyl amino group and the electron‐accepting PCB skeleton through a series of π−spacers allowed us to achieve NIR fluorescence. Particularly, a dye with a thienylisothianaphthene spacer showed NIR fluorescence with a significant intensity (λ PL =822 nm, Φ PL =0.19 in DMSO). Detailed experimental and theoretical analyses suggested that the structural relaxation to a planar quinoidal form in the S 1 state would be critical for intense NIR fluorescence.